The present invention relates to an optical transmitter-receiver in an optical communications system using optical fibers, and particularly to an optical transmitter-receiver suitable for a system employing plastic optical fibers. In addition, the present invention relates to a mechanism for preventing the adverse effects of laser light from the optical transmitter-receiver on humans.
FIG. 13 is an explanatory diagram showing a conventional fiber-optic communications system. As shown in FIG. 13(a) the system includes optical transmitter-receivers 101 and 102 linked by optical fibers 103 and 104. An optical signal 105 is transmitted from the optical transmitter-receiver 101 to the optical transmitter-receiver 102 via the optical fiber 103. Similarly, an optical signal 106 transmitted from the optical transmitter-receiver 102 is transferred to the optical transmitter-receiver 101 via the optical fiber 104. This fiber-optic communications system is referred to as a point-to-point type.
FIG. 13(b) shows a shared bus type communications system. This system comprises optical transmitter-receivers 111, 112, 113, and 114 and a passive optical splitter 115. Optical signals transmitted from the optical transmitter-receiver 111 are transferred to the optical transmitter-receivers 112 through 114 after being branched by the optical splitter 115. Optical signals transmitted from the optical transmitter-receivers 112 through 114 are merged by the optical splitter 115 and transferred to the optical transmitter-receiver 111. In actual operations, the system uses a time-sharing method to prevent the optical transmitter-receivers 112 through 114 from transmitting optical signals simultaneously. The optical splitter 115 can be thought of as a device functioning to combine the optical signal channels.
FIG. 14 is a timing chart illustrating how optical signals are transferred in a conventional fiber-optic communications system. FIG. 14(a) shows the state of optical signal transmission in a fiber-optic communications system using the point-to-point method, as shown in FIG. 13(a). As shown in the diagram, valid data 122 and 124 are transmitted when available, and idle signals 121 and 123 are transmitted when there is no valid data. During normal operations, therefore, some type of optical signal is being exchanged between the two optical transmitter-receivers 101 and 102 at all times.
FIG. 14(b) is a time chart for the state of optical signals in the shared bus optical communications system of FIG. 13(b). In this method, valid data 125 and 126 are transmitted, but idle signals are not transferred therebetween. Therefore, there are periods in which no optical signals are being transferred.
The above-described optical transmitter-receivers do not give rise to problems when properly connected by cables. However, if an optical transmitter-receiver 131 is not connected to an optical fiber, as shown in FIG. 15(a), a laser light 132 is emitted from the optical transmitter-receiver 131 into free space. Such laser light 132 can have an adverse effect on a human eye 133. The laser light can adversely affect the human eye even when a cover or the like is provided over the optical fiber connection point on the optical transmitter-receiver. For example, when an optical fiber 135 is connected on one end to the optical transmitter-receiver 131 and while the other end is open to free space, as shown in FIG. 15(b), a laser light 136 is emitted into free space from the open end of the optical fiber 135 and can cause harm to the human eye 133. Conventionally, the output of the laser light from the optical transmitter-receiver has been limited to avoid harming the human eye. In other words, the optical transmitter-receiver is designed to prevent effects on the human eye even when the laser light is emitted into free space.
With the increasing transfer rates of laser light, however, a high output laser light is becoming more necessary to enable proper reception of the laser signals that decay from long-distance transfers through optical fibers. Recently, a graded index plastic optical fiber (GI-POF) was developed. This optical fiber is far less expensive than conventional quartz optical fibers and has a broad transmission band, which is thought to be suitable for application to local area networks (LAN) and communication networks of information appliances. At this stage, however, the GI-POF still has a greater loss than quartz optical fibers and cannot be transmitted reliably over long distances.
As a safety measure, a mechanism has been proposed in U.S. Pat. No. 5,428,471 that automatically shuts down operations of an optical amplifier locally when there is a disruption in the optical fiber and automatically restores operations when the connection has been restored.
In view of the foregoing, it is an object of the present invention to provide a mechanism that improves the transmission output of an optical transmitter-receiver suitable for a point-to-point fiber-optic communications system, while preventing adverse effects on the human eye.
These objects and others will be attained by an optical transmitter-receiver for use in a point-to-point fiber-optic communications system connected by optical fibers. The optical transmitter-receiver comprises a dummy signal generator; a detector for detecting an optical signal received from another optical transmitter-receiver; a switch for switching between a dummy signal and a normal signal and transmitting a dummy signal when an optical signal is detected from another optical transmitter-receiver; and a mechanism for changing the transmission power and reducing the transmission power when an optical signal is detected from another optical transmitter-receiver.
An optical transmitter-receiver for point-to-point optical transmission and having the construction described above can prevent harm to the human eye by laser light emitted into free space when an optical fiber connection is broken. Further, the transmitter-receiver can automatically resume proper transmission when the optical fiber is properly reconnected.
The dummy signal generator can be provided either separately from a mechanism for outputting normal signals or together with the same. The above features and others will be described in more detail below within the scope of the attached claims.